UnitedHealth is being sued by the US government over Medicare charges

The U.S. Justice Department has joined a whistleblower lawsuit against UnitedHealth Group Inc. that claims the country's largest health insurer and its units and affiliates overcharged Medicare hundreds of millions of dollars, a law firm representing the whistleblower said on Thursday.

"We reject these more than five-year-old claims and will contest them vigorously," UnitedHealth spokesman Matthew Burns said in a statement.

The lawsuit, filed in 2011 and unsealed on Thursday, alleges UnitedHealth Group overcharged Medicare by claiming the federal health insurance program's members nationwide were sicker than they were, according to the law firm Constantine Cannon LLP.

The Justice Department has also joined in allegations against WellMed Medical Management Inc., a Texas-based healthcare company UnitedHealth bought in 2011.

The lawsuit by whistleblower Benjamin Poehling, a former UnitedHealth executive, has been kept under seal in federal court in Los Angeles while the Justice Department investigated the claims for the past five years. Constantine Cannon posted the lawsuit online when it was unsealed on Thursday.

No total damages were specified in the lawsuit.

(Reporting by Akankshita Mukhopadhyay and Laharee Chatterjee in Bengaluru; editing by Lisa Shumaker)